The invention relates generally to boiling water reactors and more particularly to a boiling water reactor design which effectively eliminates the possibility of reactor core meltdown.
Efforts to provide a nuclear power reactor with greatly enhanced safety features have led to a pressurized-water reactor concept which is designated the PIUS/pressurized-water reactor, where PIUS refers to a set of design criteria and is an acronym for Process Inherent Ultimate Safety. The design criteria for the PIUS reactor are safe reactor shutdown and afterheat cooling for a period in excess of one week under various adverse conditions where the shutdown and emergency core cooling systems of conventional pressurized-water reactors could be rendered inoperative. In practice, such design criteria imply that there will be no electrical or moving mechanical components such as valves, pumps, etc. required for safe reactor shutdown and emergency core cooling since such components are subject to misoperation, malfunction or sabotage. Such a design has the potential advantages of minimizing public concern for reactor safety, reducing siting restrictions, lowering licensing requirements and shortening construction schedules.
Design features of PIUS reactors include a large prestressed concrete reactor vessel (PCRV) which contains the reactor, pumps and steam generators in the case of a pressurized-water reactor. All openings are at the top of the PCRV which is sized to hold an inventory of water sufficient to cool the reactor by boiling upon shutdown for a period exceeding one week, thus obviating the need for a conventional emergency core cooling system with external pumps, piping systems and makeup water. Contained inside the PCRV are separate volumes of (1) water coolant, which has a low boron content for reactor reactivity control, for cooling the reactor during normal operation and (2) emergency water coolant which is highly borated to shut down the reactor and cool it if it enters the core. In effect, a pressurized-water reactor is submerged in a large pool of borated water which flows into the core, shutting it down and providing afterheat cooling, when conditions permit such entry. All major safety-related components are within the PCRV and protected by the PCRV which, because of its semiunderground location, its design with all major openings at or near the top of the vessel, its strength and its mass, is capable of withstanding attack by conventional munitions and severe natural disturbances without loss of coolant.
The two water volumes are in direct contact at two or more points with no valves or other obstructions to prevent flow of the highly borated water into the reactor core under emergency conditions. During normal operation, the hot low-density, low-boron-content normal cooling water is maintained above the cool, high-density, highly borated emergency coolant water at each interface in a naturally stable stratification which minimizes mixing of the two volumes. In addition, a pressure balance is maintained at the upper and lower interface through: (1) the reactor core and riser; (2) the cool borated water; and (3) the steam generators and pumps. Pressure drops across the steam generators and pumps are compensated for through the use of variable-speed recirculative pumps with pump speed being controlled by means of temperature sensors which monitor the interface between the hot and cold water volumes. Failure or misoperation of the recirculative pumps, steam generator feedpump failure, boiling in the core or other core thermal disturbances or flow disruptions create conditions under which a water circulation pattern is established whereby cool highly borated water enters the core to shut it down and continues to circulate through the core to remove afterheat due to thermal convection and boiling without the need for pump operation.
Although the PIUS design concept has been heretofore limited to pressurized-water reactors, application of that concept to boiling-water reactors is desirable for the same safety-related reasons advanced in connection with pressurized-water reactors. However, such application is complicated in boiling-water reactors by the existance of significant variations in system pressure and component pressure drops with variations in reactor power. These variations make the hydrostatic balancing approach as used in the PIUS pressurized-water reactors unusable in a PIUS boiling-water reactor.
It is, accordingly, a general object of the invention to provide a PIUS boiling-water reactor design.
Another object of the invention is to provide any accident situation a means for ensuring circulation of cool highly borated water through a boiling-water reactor core in a reactor incorporating PIUS design principles.
Other objects and benefits of the invention will become apparent to those skilled in the art upon examination of the following detailed description of a preferred embodiment of the invention and the accompanying drawings.